Billable duration is the build and post-build time combined, rounded up to the next whole minute. CPU minutes are that figure multiplied by the machine's vCPUs.
To see the same breakdown from the CLI, update to version 59.23.1 or later with npm i -g vercel@latest, then run vc inspect:
1
vc inspect
3 Duration
5 build duration 5m 9s
6 post-build duration 3m 21s
7 billable duration 9m
8 CPU Minutes Usage 270 minutes (9m x 30vCPU)
Learn more about build machines and CPU minutes in the docs.
